1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more particularly to a lithographic projection apparatus including a system for suppression of contamination of the optical system.
2. Background of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In the case of the current invention, the projection system will generally consist of an array of mirrors, and the mask will be reflective; see, for example, the apparatus discussed in WO 99/57596. The radiation in this case is preferably electromagnetic radiation in the extreme ultraviolet (EUV) range, which may be produced by a plasma source. Typically, the radiation has a wavelength below about 50 nm, preferably below about 20 nm and most preferably below about 15 nm. An example of a wavelength in the EUV region which is gaining considerable interest in the lithography industry is 13.4 nm, though there are also other promising wavelengths in this region, such as 11 nm, for example.
An example of the radiation system suitable for use with such radiation is described in WO 00/36471. Such a radiation system may comprise a suitable condenser for use with EUV as described in EP 1037113.
It is well known that contaminant particles, such as hydrocarbon molecules and water vapor, are present in lithographic projection apparatus. These contaminant particles may include debris and by-products that are sputtered loose from the substrate, for example by an EUV radiation beam. Said particles may also include debris from the EUV source, contaminants liberated at actuators, conduit cables, etc. Since parts of lithographic projection apparatus, such as the radiation system and the projection system, are generally at least partially evacuated, these contaminant particles tend to migrate to such areas. The particles then adsorb to the surfaces of the optical components located in these areas. This contamination of the optical components causes a loss of reflectivity, which may adversely affect the accuracy and efficiency of the apparatus, and may also degrade the components"" surfaces, thus reducing their useful lifetime.
Previous measures which have been proposed to remove impurities from the apparatus have involved suppressing the movement towards the projection system of debris produced by the sputtering of the surface of the substrate. For example, it has previously been suggested that the problem can be addressed by increasing the distance between the substrate and the final optical component, by introducing a xe2x80x9cgas curtainxe2x80x9d between the substrate and the final optical component to sweep away the debris. Such measures are directed to preventing contaminants from entering, for instance, the projection system. However, some contaminants may still enter the system or may be generated in the system itself, e.g. by moving parts.
One aspect of embodiments of the present invention provides a lithographic apparatus in which contamination of optical components is suppressed thereby addressing the problems of the previously used systems as described above.
According to one embodiment of the invention there is provided a lithographic projection apparatus wherein that a space containing at least a part of a radiation system, and/or a space containing at least a part of a projection system contains an inert gas at a pressure of 0.1 to 10 Pa.
By introducing an inert gas into either or both of the radiation or the projection system, the mean free path of any contaminant particles contained in the system is reduced. This has the effect of suppressing the contamination of any optical components, such as mirrors, located in the radiation and/or projection systems. The contamination of such optical components can be suppressed by a factor of about 10 to 103 using this method. Thus, the mirror surfaces are protected and the decrease in their reflectance over time is reduced, thereby increasing their useful lifetime.
The invention also relates to a method of manufacturing a device using a lithographic projection apparatus including providing a substrate that is at least partially covered by a layer of radiation-sensitive material, providing a projection beam of radiation using a radiation system, using patterning structure to endow the projection beam with a pattern in its cross-section, projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, and supplying an inert gas to a space containing at least a part of said radiation system and/or a space containing at least a part of said projection system, wherein the pressure in said space(s) is from 0.1 to 10 Pa.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d (Z) used herein is intended to refer to the direction normal to the substrate or mask surface, rather than implying any particular orientation of the apparatus.